A servo system is used in a driving mechanism for driving an object to be controlled such as a robot and the like, and it is desirable in this case that torque applied to the object to be controlled be regulated. For that purpose, generally a torque detecting device is provided between a reduction unit composing a driving force transmission mechanism and an object to be controlled in order to detect transmission torque. For an appropriate torque detection, it is desirable that a reduction unit is provided therein with a torque detecting mechanism. In such a torque detecting mechanism, there have known a method of using a strain gauge, and that of measuring impedance of a magnetic-material-coated rotational part of a reduction unit.
As a reduction unit accommodated in a servo system, a flexible meshing type gear is known. The flexible meshing type gear is widely used in a system requiring drive control with accuracy, because the device of this type has a small number of parts and can give high reduction gear ratio. As a torque detecting mechanism for the flexible meshing type gear, there is a mechanism in which a strain gauge is adhered on a component flexible external gear. There is also a mechanism in which an outer periphery of a flexible external gear is coated with a magnetic material, and at the same time a magnetic head is placed in a position facing the coated position.